The present invention relates to an electric power steering apparatus.
An electric power steering apparatus (EPS) using a motor as a driving source is generally employed as a power steering apparatus of a vehicle. In many cases, the EPS performs steering wheel return compensation control as one of compensation controls so that a steering wheel is returned to a neutral position (see, for example, Japanese Laid-Open Patent Publication No. 2002-145101).
Theoretically, as long as the vehicle is moving, the steering wheel is returned to the neutral position (that is, the position at which the steering angle becomes zero) by the self aligning torque acting on the steered wheels, without operation of the steering wheel. However, if friction in the system of the EPS, which is, for example, a decelerating mechanism including a ball screw and a worm and a wheel, exceeds the self aligning torque acting on the steered wheels, the steering wheel cannot be fully returned to the neutral position. In this case, the steering wheel return compensation control is carried out so that the steering wheel is quickly returned to the neutral position. The control involves calculation of a steering wheel return compensation amount in accordance with which the steering wheel is rotated to restore its neutral position. The compensation amount is superimposed on a basic assist control amount, which is a basic component of power assist control.
However, the self aligning torque, which acts on the steered wheels, changes depending on the condition of the road surface. Thus, in the case of the conventional EPS, particularly if the vehicle is traveling on a low-μ road such as an icy road, the self aligning torque may decrease and thus hamper returning of the steering wheel. In other words, the conventional EPS is designed in correspondence with a general condition of a road surface on which the vehicle travels for a usual purpose of use, that is, for traveling on a high-μ road such as a paved road. Further, the steering wheel return compensation control is designed in correspondence with the self aligning torque produced by traveling on a high-μ road. Thus, when the vehicle travels on a low-μ road, the steering wheel return compensation amount may become insufficient. In this regard, the conventional EPS has yet to be improved.